1. Field of the Invention
This invention relates to a silicon oxide depositing source and more particularly, a silicon oxide depositing source which lends itself to vacuum deposition by electron beam heating to form a silicon oxide thin film having transparency and barrier properties in high yields. It also relates to a plastic film having such a silicon oxide coating deposited thereon.
2. Prior Art
Transparent barrier films are manufactured by evaporating and depositing a silicon oxide thin coating on plastic web as disclosed in Japanese Patent Publication (JP-B) No. 12953/1978 and Japanese Patent Application Kokai (JP-A) No. 253434/1989. Induction heating and resistance heating techniques are conventional for such evaporation. Recently, sputtering and electron beam heating techniques are employed for the purpose of improving film properties. The depositing source also experiences a transition from silicon monoxide alone to a mixture of metallic silicon and silicon dioxide partly because of low cost.
Among the evaporating techniques, the electron beam heating technique has the advantage of achieving efficient evaporation by locally concentrating the energy of an electron beam, but, as described in JP-A 15526/1991, suffers from a lowering of vacuum and an unstable deposition rate upon evaporating metal oxide. As the power of an electron beam is increased the beam can be disturbed due to charge build-up of electrons and grains of the deposition source can be splashed as such without being vaporized, which is known as a splash phenomenon. These deteriorate the film quality.
One countermeasure is to mix a metal such as magnesium and aluminum with the metal oxide. When a mixture of metallic silicon and silicon dioxide was evaporated on a transparent plastic web, it was difficult to produce a thin film which satisfied both full colorless transparency and barrier properties.
For these reasons, the electron beam heating technique as applied to the evaporation of a mixture of metallic silicon and silicon dioxide is rather low in deposition rate and hence, productivity as compared with the resistance heating and induction heating techniques.